Geotechnical - Load and Resistance Factor Design

By

Mir Zaheer, P.E.,Geotechnical Engineer ,

INDOT Office of Geotechnical Engineering

INDOT Structures Conference - 2010

MSE Walls

AASHTO Section 11FHWA GEC 11

FHWA-NHI-10-024 & 25

MSE Walls

In LRFD, the external and internal stability of the MSE wall is evaluated at all appropriate Limit states.

In the AASHTO-LRFD framework, there are four limit states, which represent distinct structural performance criteria: (1) strength limit states (2) Serviceability limit states (3) extreme event limit states (4) fatigue limit states

For most earth retaining system designs, the strength or service limit states control the design. For walls subject to earthquake or vessel/vehicle impact, the extreme limit states may control.

Design of MSE walls Using LRFD Methodology

Strength Limit States External Stability

Limiting Eccentricity Sliding Bearing Resistance

Internal Stability Tensile Resistance of

Reinforcement Pullout Resistance of

Reinforcement Structural Resistance of

face elements Structural resistance of

Face Element Connections

Service Limit States External Stability Vertical WALL Movements Lateral Wall Movements

Global Stability Overall Stability Compound Stability

Internal Stability is the responsibility of the manufacturer

LRFD for MSE Walls

Load Combinations Load Factors for Permanent Loads External Stability Resistance Factors

Loads and Combinations

Permanent Loads EH = Horizontal Earth Loads ES = Earth Surcharge Load EV = Vertical Pressure from

dead load of earth fill

Transient Loads CT = Vehicular collision force EQ = Earthquake Load LL = Vehicular Live Load LS = Live Load Surcharge

Load Combination Limit State

EHESEV

LLLS EQ CT

Strength I γP 1.75 -- --

Extreme Event I γP γEQ 1.0 --

Extreme Event II γP 0.5 -- 1.0

Service I 1.0 1.0 -- --

γp =Load factor for permanent loading. May subscript as γP-EV, γP-EH

γEQ = Load factor for live load applied simultaneously with seismic loads

Table 3.4.1-1 AASHTO 2007

Load Factors for Permanent Loads, γP

Table 3.4.1-2 AASHTO 2007

Type of Load Load Factor, γP

Maximum Minimum

DC: Component and Attachments 1.25 0.90

EH: Horizontal Earth PressureActive 1.50 0.90

EV: Vertical Earth Pressure Overall Stability Retaining Walls and Abutments

1.001.35

N/A1.00

ES: Earth Surcharge 1.50 0.75

Note: May subscript as γEV-MAX γEV-MIN, γEH-MAX, γEH-MIN, ect.

External Stability Resistance Factors for MSE Walls

Stability Mode Conditions Resistance Factor, Φτ

Bearing Resistance None 0.65

Sliding None 1.0

Overall (Global) Stability

Where geotechnical parameters are well defined, and the slope does not support or contain a structural element

0.75

Where geotechnical parameters are based on limited information, or the slope contains or supports a structural element

0.65

MSE Wall Design Example

MSE Wall Design Example

MSE Wall Design Example

MSE Wall Design Example

MSE Wall Design Example

MSE Wall Design Example

CDR = Capacity To Demand Ratio

MSE Wall Design Example

MSE Wall Design Example

If the sliding, capacity demand ratio, CDR < 1, INCREASE the reinforcement length, L, and repeat the calculations.

General Foundation Design Flow Chart1. Establish Global Project Performance Requirements and Constraints

2. Define Preliminary Project Geotechnical Site Conditions

3. Determine Substructure Loads and Load Combinations at Foundation Level

4. Develop and Execute Subsurface Exploration and Laboratory Testing Program for Feasible Foundation System

5. Evaluate Information and Determine Foundation Systems for Further Evaluation

6. Deep Foundations Shallow Foundations

Without Ground Improvement

With Ground Improvement

Shallow Foundations

AASHTO SECTION 10.6FHWA – RC/TD -10-001

FHWA-NHI-05-094

Shallow Foundation Design Flowchart

Define Subsurface Conditions and any

Geometric Constraints

Determine depth of footing based on

geotechnical bearing, scour, and frost

protection considerations

Determine Applicable Loads and Load Combinations

Factor Loads for Each Combination

Determine Design Soil Properties &

Resistance Factors

Check Global Stability at the Service Limit

State

Determine the Nominal Geotechnical Bearing

Resistance at the Service Limit State

Size the Footing at the Service Limit State

Determine the Nominal Bearing & Sliding Resistance at the

Strength & Extreme Limit States

Check Footing at Strength Limit State for Bearing, Sliding

and Eccentricity

Check Footing at Strength & Extreme

Limit States for Sliding, Eccentricity and Bearing Stress

Perform Structural Design of Footing Based on Factored Loads and Factored

Resistances

Limit States for Spread Footings

Design of a spread footing must provide adequate resistance against geotechnical and structural limit states, i.e., "failure“ modes. The geotechnical limit states includethe following: Strength limit state

Bearing resistance Limiting eccentricity Sliding

Strength Limit States

Sliding Limiting Eccentricity

Bearing Resistance

Service limit state Settlement Global stability

Extreme Event limit state Bearing resistance Limiting eccentricity Sliding

Service Limit States

Settlement Overall Stability

The structural design includes considerationof limit states for the following: • Flexural resistance (strength limit) • Shear resistance (strength limit) • Crack control (service limit)

Horizontal Deformations-Settlements & Rotations

Bearing resistance Chart

Design Resources

LRFD DESIGN - BASICS

Load factor combinations to obtain resulting maximum force effects on the foundations are needed for limit states checks.

This is done through structure modeling by varying the load factors over the specified range

Service Limit State I-Checks

Vertical deformation – Settlement Horizontal movements at the top of

foundation Rotations at the top of foundation Vertical and horizontal deformations under

scour at the Design flood, Q100 Settlements due to downdrag(AASHTO 10.5, 10.6, 10.7 & 10.8)

Requirements for LRFD Design

Tolerable vertical and horizontal deformations (movements) are established by the structural designer, based on structural tolerance to total and differential movements, rideability, and economy. (AASHTO 10.5.2.1)

Strength Limit States - Checks

Geotechnical lateral resistance of soil and rock Geotechnical axial compression resistance Geotechnical axial uplift resistanceAll the above for single and group foundations Structural resistance checks for axial, lateral and flexure Punching of foundation elements through stronger soil in

to weaker soilsAll the above resistances under scour at design flood, Q100 Axial resistance when downdrag occurs

(AASHTO 10.5, 10.6, 10.7 & 10.8)

Axial Geotechnical Resistance

Overall Stability

Methods for determining structural resistance Axial compression Combined axial and flexure Shear

Concrete – AASHTO Section 5 Steel – AASHTO Section 6

Structural Axial Failure Mode

Structural Flexure Failure Mode

Structural Shear Failure Mode

Load & Resistance Factor Design:

Deep Foundations (AASHTO 10.7 & 10.8) Piles Drilled Shafts

Shallow Foundations (AASHTO 10.6) Spread Footings

MSE Walls & Other Retaining Walls (AASHTO 11)

Culverts, Tunnels and other buried Structures (AASHTO 11)

Deep Foundations

Driven Piles - AASHTO 10.7Drilled Shafts - AASHTO 10.8

FHWA-NHI-05-094FHWA-NHI-10-016

6. Deep Foundations

7. Select Driven Pile Foundations for Further Evaluation

8. Select Static Analysis Method and Calculate Ultimate Axial Capacity vs. Depth

9. Identify Most Economical Pile Types from Ultimate Capacity vs. Depth Charts

10. Drivability of pile types to penetration depths and sufficient ultimate capacities

11. Select pile types, ultimate capacities, and pile penetration depths for group sizing

12. Evaluate Group axial, Lateral, and Rotational Capacities, Settlement, and performance of pile group configuration

Evaluate Other Deep Foundation Systems

i.e. Drilled Shafts

General Deep Foundation Design Flow Chart for Driven Piles

Geotechnical Resistance Factors for Driven Piles

Condition/Resistance Determination Method

AASHTO Resistance

Factor

ModifiedResistance

Factor

Nominal Resistance of Single Pile in Axial Compression – Dynamic Analysis and Static Load Test Methods,Φdyn

Static Load Test in combination w/ dynamic testing or wave equation

0.8 0.8

PDA w/CAPWAP .65 0.7

Gates Formula 0.4 0.55

Geotechnical Design Memos

GDM 2010-01 Memo listing the Maximum

Nominal Soil Resistance for Common Pile Types

Nominal soil resistance based on modified resistance factors

Available on the Internet

Pile Section

Pile Area(in2)

Rn max (Kips)

10x42 HP 12.4 341

10x57 HP 16.8 462

12x53 HP 15.5 426

12x63 HP 18.4 506

12x74 HP 21.8 600

12x84 HP 24.6 677

14x73 HP 21.4 589

14x89 HP 26.1 718

14x102 HP 30.0 825

14x117 HP 34.4 946

14" Pipe pile SEC

420

16" Pipe pile SEC

480

Maximum Nominal Soil Resistance for Common Piles to Rock

Horizontal Displacement (P-y method)

Ht

Qt

MtyP

y

yPropertiesA, E, I

P

y

Pm * P

P

Spacing (S) Row 1 Row 2 Row 33D 0.7 0.5 0.355D 1.0 0.85 0.7

P-multiplier (Pm)D

S

From Table 10.7.2.4-1

Example Pile Design -Pier

Example Pile Design -Pier

EXAMPLE PILE LOADS

Strength Extreme Values in Pile Load Table

Factored Loads QF

200 K 300 K 210 K

Resistance Factor φ

0.7 1.0 0.7

Nominal Soil Resistance Rn

286 K 300 K 300 K

The extreme loads control the design. Hence the pile shall be driven to a nominal capacity that provides the required controlling factored loads.

Limitations of Resistance Factors:AASHTO LRFD BRIDGE DESIGN SPECIFICATIONS 10.5.5.2.3

The selection of the target reliability assumes a significant amount of redundancy in the foundation system is present, which is typical for pile groups containing at least five piles in the group. For smaller groups and single piles, less redundancy will be present. These smaller pile groups that lack redundancy. Therefore, the resistance factors specified in Table 1 should be reduced to account for reduced redundancy.

Drilled Shafts

AASHTO 10.7 AND 10.8FHWA GEC 10

FHWA-NHI-10-016

6. Deep Foundations

7. Select Drilled Shaft Foundations for Further Evaluation

8. Define Subsurface Profile for analysis

9. Determine Resistance Factors for Design

10. Establish Minimum Diameter and Depth for Lateral Loads

11. Establish Diameter and Depth for Axial Loads

12. Finalize Structural Design of the Drilled Shafts and Connection to Structure (or cap)

Evaluate Other Deep Foundation Systems i.e.

Piles

General Deep Foundation Design Flow Chart for Drilled Shafts

10. Establish Minimum Diameter and Depth for Lateral Loads

10.1 Refine Detailed Subsurface Profiles as needed for each Lateral Load Case, including scour, liquefaction, fill, ect.

10.2 Select Trial Length and Diameter

10.3 Analyze Geotechnical Strength Limit State using Factored Loads (for each case)

Check Stability against Pushover Failure

Yes. 10.4 Analyze Preliminary Structural Strength Limit State for Flexure using Factored Loads

Check Moment Capacity with 1 to 2% Longitudinal Reinforcement

Yes.10.5 Analyze Service Limit State (Deformations) using Unfactored Loads

Check: Deformations Acceptable

Yes. 10.6 Define Minimum Pile Length and Diameter based on analysis No . Return to 10.2 and Revise Design

No. Revise Diameter and repeat steps starting at 10.2

No. Revise Length and repeat steps starting at 10.2

Lateral Loads Design Process For Drilled Shafts

11.1 Idealized Geomaterial Layer Profiles

11.2 Review Limit States and Factored Axial Force Effects

11.3 Assign Appropriate Geomaterial Properties to each Subsurface Layer

11.4 Select Trial Lengths and Diameters

11.5 Establish Nominal Side and Base Resistances

11.6 Evaluate Trial Design for LRFD Strength Limit States

Yes. 11.7 Evaluate Trial Design for LRFD Service Limit States

Yes. Design Complete No. Return to 11.4 and Redesign

No. Return to 11.4 and Redesign

Establish Minimum Depths and Diameters for Axial Loads for Drilled Shafts

Geotechnical Resistance FactorsDrilled Shafts

Method φComp φTen

α - Method (side) 0.55 0.45β - Method (side) 0.55 0.45Clay or Sand (tip) 0.5Rock (side) 0.55 0.45Rock (tip) 0.55Group (sand or clay) 0.55 0.45Load Test 0.7

AASHTO Table 10.5.5.2.3-1

Summary of Resistance Factors for LRFD Design of Drilled Shaft Foundations

Drilled Shaft Resistance in Rock

Side Resistance

Tip Resistance

Total ResistanceA

BCD

Qb

QS

QR = φQn = φqbQb + φqsQs

Displacement

Res

ista

nce

Questions?

EXAMPLE CALCSLimit State Nominal

Load (kip)ResistanceFactor (φ)

Factored Load (kip)

MaximumService I

183.6 1.00 183.6

Maximum Strength I

253.2 0.70 177.5

Maximum Extreme I

356.9 1.00 356.9

Extreme I is controlling. The pile is driven to this maximum nominal load in the pile load Table